Effect of adrenaline on the post-tetanic potentiation in mouse skeletal muscle

Force-frequency relationship during fatiguing contractions of rat medial gastrocnemius muscle

The force–frequency relationship presents the amount of force a muscle can produce as a function of the frequency of activation. During repetitive muscular contractions, fatigue and potentiation may both impact the resultant contractile response. However, both the apparent fatigue observed, and the potential for activity-dependent potentiation can be affected by the frequency of activation. Thus, we wanted to explore the effects that repetitive stimulation had on the force–frequency relationship. The force–frequency relationship of the rat medial gastrocnemius muscle was investigated during consecutive bouts of increasing fatigue with 20 to 100 Hz stimulation. Force was measured prior to the fatiguing protocol, during each of three levels of fatigue, and after 30 min of recovery. Force at each frequency was quantified relative to the pre-fatigued 100 Hz contractions, as well as the percentage reduction of force from the pre-fatigued level at a given frequency. We observed less reduction in force at low frequencies compared to high frequencies, suggesting an interplay of fatigue and potentiation, in which potentiation can “protect” against fatigue in a frequency-dependent manner. The exact mechanism of fatigue is unknown, however the substantial reduction of force at high frequency suggests a role for reduced force per cross-bridge.

Cycling Performance Enhancement After Drop Jumps May Be Attributed to Postactivation Potentiation and Increased Anaerobic Capacity

de Poli, RAB, Boullosa, DA, Malta, ES, Behm, D, Lopes, VHF, Barbieri, FA, and Zagatto, AM. Cycling performance enhancement after drop jumps may be attributed to postactivation potentiation and increased anaerobic capacity. J Strength Cond Res 34(9): 2465-2475, 2020-The study aimed to investigate the effects of drop jumps (DJs) on supramaximal cycling performance, anaerobic capacity (AC), electromyography, and fatigue. Thirty-eight recreational cyclists participated into 3 independent studies. In study 1 (n = 14), neuromuscular fatigue was assessed with the twitch interpolation technique. In study 2 (n = 16), the AC and metabolic contributions were measured with the maximal accumulated oxygen deficit method and the sum of the glycolytic and phosphagen pathways. In study 3 (n = 8), postactivation potentiation (PAP) induced by repeated DJs was evaluated. The DJ protocol was effective for significantly improving cycling performance by +9.8 and +7.4% in studies 1 and 2, respectively (p ≤ 0.05). No differences were observed in electromyography between conditions (p = 0.70); however, the force evoked by a doublet at low (10 Hz) and high frequencies (100 Hz) declined for control (-16.4 and -23.9%) and DJ protocols (-18.6 and -26.9%) (p < 0.01). Force decline was greater in the DJ condition (p < 0.03). Anaerobic capacity and glycolytic pathway contributions were +7.7 and +9.1% higher after DJ protocol (p = 0.01). Peak force during maximal voluntary contraction (+5.6%) and doublet evoked force at 100 Hz (+5.0%) were higher after DJs. The DJ protocol induced PAP, improved supramaximal cycling performance, and increased AC despite higher peripheral fatigue.

Post-activation Potentiation Versus Post-activation Performance Enhancement in Humans: Historical Perspective, Underlying Mechanisms, and Current Issues

Post-activation potentiation (PAP) is a well-described phenomenon with a short half-life (~28 s) that enhances muscle force production at submaximal levels of calcium saturation (i.e., submaximal levels of muscle activation). It has been largely explained by an increased myosin light chain phosphorylation occurring in type II muscle fibers, and its effects have been quantified in humans by measuring muscle twitch force responses to a bout of muscular activity. However, enhancements in (sometimes maximal) voluntary force production detected several minutes after high-intensity muscle contractions are also observed, which are also most prominent in muscles with a high proportion of type II fibers. This effect has been considered to reflect PAP. Nonetheless, the time course of myosin light chain phosphorylation (underpinning “classic” PAP) rarely matches that of voluntary force enhancement and, unlike PAP, changes in muscle temperature, muscle/cellular water content, and muscle activation may at least partly underpin voluntary force enhancement; this enhancement has thus recently been called post-activation performance enhancement (PAPE) to distinguish it from “classical” PAP. In fact, since PAPE is often undetectable at time points where PAP is maximal (or substantial), some researchers have questioned whether PAP contributes to PAPE under most conditions in vivo in humans. Equally, minimal evidence has been presented that PAP is of significant practical importance in cases where multiple physiological processes have already been upregulated by a preceding, comprehensive, active muscle warm-up. Given that confusion exists with respect to the mechanisms leading to acute enhancement of both electrically evoked (twitch force; PAP) and voluntary (PAPE) muscle function in humans after acute muscle activity, the first purpose of the present narrative review is to recount the history of PAP/PAPE research to locate definitions and determine whether they are the same phenomena. To further investigate the possibility of these phenomena being distinct as well as to better understand their potential functional benefits, possible mechanisms underpinning their effects will be examined in detail. Finally, research design issues will be addressed which might contribute to confusion relating to PAP/PAPE effects, before the contexts in which these phenomena may (or may not) benefit voluntary muscle function are considered.

Epinephrine augments posttetanic potentiation in mouse skeletal muscle with and without myosin phosphorylation

Sympathetic tone may influence force potentiation, that is, the stimulation-induced increase in skeletal muscle mechanical function associated with myosin phosphorylation, although the mechanism for this effect remains unknown. The purpose of this study was to examine the influence of epinephrine on concentric twitch force potentiation of wild-type and skeletal myosin light-chain kinase devoid mouse muscle (skMLCK-/- ). To this end, concentric twitch force was assessed before and after a potentiating stimulus (PS) to determine the peak and the duration of potentiation in the absence (-EPI) and presence (+EPI) of 1 μmol/L epinephrine in both genotypes. Twitch force of wild-type and skMLCK-/- muscles was increased by up to 31 and 35% and 18 and 23% in the -EPI and EPI conditions, respectively (all data n = 8, P < 0.05). In wild-type muscles, the PS increased RLC phosphorylation from 0.14 ± 0.05 (rest) to 0.66 ± 0.08 mol phos mol RLC; by 480 sec RLC phosphorylation had returned to baseline (all data n = 4 each time point, P < 0.05). Neither resting nor peak levels of RLC phosphorylation were altered by +EPI, although the duration of RLC phosphorylation was prolonged. In skMLCK-/- muscles, RLC phosphorylation was not elevated above constituent levels by stimulation in either the -EPI or +EPI condition. Thus, given the similarity in potentiation responses between genotypes our data suggest that the influence of epinephrine on potentiation was independent of skMLCK catalyzed phosphorylation of the RLC, although the clinical significance of this pathway for skeletal muscle function remains to be identified.

Post-activation potentiation (PAP) in endurance sports: A review

While there is strong support of the usefulness of post-activation potentiation (PAP) phenomenon in power demanding sports, the role that PAP could play in endurance sports has received less attention. The aim of this review is to present evidence for a better understanding of PAP in endurance athletes; and to discuss the physiological basis and methodological aspects necessary for better practices and designing further studies. A search for relevant articles on PAP and endurance trained athletes was carried out using Medline and ISI Web of Knowledge databases. Twenty-two studies were included in the review. The current evidence suggests the possible influence of PAP for performance enhancement after appropriate conditioning activities during warm up. Evaluation of PAP responses during testing, training and competition may be also important for athletes monitoring. There are many unresolved questions about the optimum load parameters for benefiting from PAP in both training and competition; and the role that PAP may exert for optimal performance while interacting with central and peripheral factors associated with muscle fatigue. Further studies should elucidate the association between PAP responses and long-term adaptations in endurance athletes.

Nonlocalized postactivation performance enhancement (PAPE) effects in trained athletes: a pilot study

Fifteen trained athletes were assessed for postactivation performance enhancement (PAPE) of squat jumps (SJs) and power push-ups (PPUs) following upper body activation, lower body activation, upper and lower body activation, and rest. SJ improved similarly across all 4 conditions. PPU could not be assessed. Since the test protocol of SJ and PPU involved upper and lower body activation and caused PAPE in SJ, future work is required to determine if a nonlocalized PAPE effect exists.

Modulation of Skeletal Muscle Contraction by Myosin Phosphorylation

The striated muscle sarcomere is a highly organized and complex enzymatic and structural organelle. Evolutionary pressures have played a vital role in determining the structure-function relationship of each protein within the sarcomere. A key part of this multimeric assembly is the light chain-binding domain (LCBD) of the myosin II motor molecule. This elongated "beam" functions as a biological lever, amplifying small interdomain movements within the myosin head into piconewton forces and nanometer displacements against the thin filament during the cross-bridge cycle. The LCBD contains two subunits known as the essential and regulatory myosin light chains (ELC and RLC, respectively). Isoformic differences in these respective species provide molecular diversity and, in addition, sites for phosphorylation of serine residues, a highly conserved feature of striated muscle systems. Work on permeabilized skeletal fibers and thick filament systems shows that the skeletal myosin light chain kinase catalyzed phosphorylation of the RLC alters the "interacting head motif" of myosin motor heads on the thick filament surface, with myriad consequences for muscle biology. At rest, structure-function changes may upregulate actomyosin ATPase activity of phosphorylated cross-bridges. During activation, these same changes may increase the Ca2+ sensitivity of force development to enhance force, work, and power output, outcomes known as "potentiation." Thus, although other mechanisms may contribute, RLC phosphorylation may represent a form of thick filament activation that provides a "molecular memory" of contraction. The clinical significance of these RLC phosphorylation mediated alterations to contractile performance of various striated muscle systems are just beginning to be understood. © 2017 American Physiological Society. Compr Physiol 7:171-212, 2017.

Myosin phosphorylation and force potentiation in skeletal muscle: evidence from animal models

The contractile performance of mammalian fast twitch skeletal muscle is history dependent. The effect of previous or ongoing contractile activity to potentiate force, i.e. increase isometric twitch force, is a fundamental property of fast skeletal muscle. The precise manifestation of force potentiation is dependent upon a variety of factors with two general types being identified; staircase potentiation referring to the progressive increase in isometric twitch force observed during low frequency stimulation while posttetanic potentiation refers to the step-like increase in isometric twitch force observed following a brief higher frequency (i.e. tetanic) stimulation. Classic studies established that the magnitude and duration of potentiation depends on a number of factors including muscle fiber type, species, temperature, sarcomere length and stimulation paradigm. In addition to isometric twitch force, more recent work has shown that potentiation also influences dynamic (i.e. concentric and/or isotonic) force, work and power at a range of stimulus frequencies in situ or in vitro, an effect that may translate to enhanced physiological function in vivo. Early studies performed on both intact and permeabilized models established that the primary mechanism for this modulation of performance was phosphorylation of myosin, a modification that increased the Ca(2+) sensitivity of contraction. More recent work from a variety of muscle models indicates, however, the presence of a secondary mechanism for potentiation that may involve altered Ca(2+) handling. The primary purpose of this review is to highlight these recent findings relative to the physiological utility of force potentiation in vivo.

Alterations of cAMP-dependent signaling in dystrophic skeletal muscle

Autonomic regulation processes in striated muscles are largely mediated by cAMP/PKA-signaling. In order to achieve specificity of signaling its spatial-temporal compartmentation plays a critical role. We discuss here how specificity of cAMP/PKA-signaling can be achieved in skeletal muscle by spatio-temporal compartmentation. While a microdomain containing PKA type I in the region of the neuromuscular junction (NMJ) is important for postsynaptic, activity-dependent stabilization of the nicotinic acetylcholine receptor (AChR), PKA type I and II microdomains in the sarcomeric part of skeletal muscle are likely to play different roles, including the regulation of muscle homeostasis. These microdomains are due to specific A-kinase anchoring proteins, like rapsyn and myospryn. Importantly, recent evidence indicates that compartmentation of the cAMP/PKA-dependent signaling pathway and pharmacological activation of cAMP production are aberrant in different skeletal muscles disorders. Thus, we discuss here their potential as targets for palliative treatment of certain forms of dystrophy and myasthenia. Under physiological conditions, the neuropeptide, α-calcitonin-related peptide, as well as catecholamines are the most-mentioned natural triggers for activating cAMP/PKA signaling in skeletal muscle. While the precise domains and functions of these first messengers are still under investigation, agonists of β₂-adrenoceptors clearly exhibit anabolic activity under normal conditions and reduce protein degradation during atrophic periods. Past and recent studies suggest direct sympathetic innervation of skeletal muscle fibers. In summary, the organization and roles of cAMP-dependent signaling in skeletal muscle are increasingly understood, revealing crucial functions in processes like nerve-muscle interaction and muscle trophicity.

Potentiation in mouse lumbrical muscle without myosin light chain phosphorylation: is resting calcium responsible?

The increase in isometric twitch force observed in fast-twitch rodent muscles during or after activity, known universally as potentiation, is normally associated with myosin regulatory light chain (RLC) phosphorylation. Interestingly, fast muscles from mice devoid of detectable skeletal myosin light chain kinase (skMLCK) retain a reduced ability to potentiate twitch force, indicating the presence of a secondary origin for this characteristic feature of the fast muscle phenotype. The purpose of this study was to assess changes in intracellular cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) after a potentiating stimulus in mouse lumbrical muscle (37°C). Lumbricals were loaded with the Ca²⁺-sensitive fluorescent indicators fura-2 or furaptra to detect changes in resting and peak, respectively, intracellular Ca²⁺ levels caused by 2.5 s of 20-Hz stimulation. Although this protocol produced an immediate increase in twitch force of 17 ± 3% (all data are n = 10) (P < 0.01), this potentiation dissipated quickly and was absent 30 s afterward. Fura-2 fluorescence signals at rest were increased by 11.1 ± 1.3% (P < 0.01) during potentiation, indicating a significant increase in resting [Ca²⁺]_i. Interestingly, furaptra signals showed no change to either the amplitude or the duration of the intracellular Ca²⁺ transients (ICTs) that triggered potentiated twitches during this time (P < 0.50). Immunofluorescence work showed that 77% of lumbrical fibers expressed myosin heavy chain isoform IIx and/or IIb, but with low expression of skMLCK and high expression of myosin phosphatase targeting subunit 2. As a result, lumbrical muscles displayed no detectable RLC phosphorylation either at rest or after stimulation. We conclude that stimulation-induced elevations in resting [Ca²⁺]_i, in the absence of change in the ICT, are responsible for a small-magnitude, short-lived potentiation of isometric twitch force. If operative in other fast-twitch muscles, this mechanism may complement the potentiating influence of myosin RLC phosphorylation.

Myosin light-chain phosphorylation and potentiation of dynamic function in mouse fast muscle

The intent of this study was to determine if the stimulation-induced increase or "potentiation" of dynamic function of mouse extensor digitorum longus muscle (in vitro 25°C) during work cycles is graded to myosin regulatory light-chain (RLC) phosphorylation. To do this, concentric force and muscle work output during sinusoidal length changes were determined before (unpotentiated) and after (potentiated) the application of conditioning stimuli (CS) producing incremental elevations in RLC phosphorylation from rest. Sine wave excursion was from 1.09 to 0.91 of L (o) with a period of 142 ms; stimulating muscles to twitch and generate force during these cycles produced plots of force × displacement termed work loops. Stimulation at 2.5-, 5.0-, and 100-Hz elevated RLC phosphorylation from 0.16±0.02 (rest) to 0.29±0.03, 0.45±0.02 and 0.56±0.02 mol phos per mole RLC, respectively (n= 6-7, P<0.05). These CS potentiated mean concentric force (at all lengths) to 1.14±0.02, 1.26±0.04 and 1.41±0.06 of pre-stimulus, control levels (all n= 5-7, P<0.05) while work was increased to 1.07±0.02, 1.17±0.02 and 1.34±0.03 of controls, respectively. In a No CS condition that did not elevate RLC phosphorylation, neither mean concentric force nor work was altered. Thus, strong correlations between RLC phosphorylation and mean concentric force and work support the hypothesis that this molecular mechanism modulates muscle power output. No length-dependence for concentric force potentiation was observed in any condition, an outcome suggesting that interactions between instantaneous variations in muscle length and shortening velocity during work cycles modulates the potentiation response.

Cellular and whole muscle studies of activity dependent potentiation

With a single activation, a skeletal muscle fiber, motor unit or whole muscle will yield a twitch contraction. The twitch is not an "all-or-none" response, but a submaximal response that can vary from one time to another. Prior activation causes myosin regulatory light chain (RLC) phosphorylation, by an enzyme called myosin light chain kinase. This phosphorylation dissipates slowly over the next several minutes due to a slow activity of light chain phosphatase. Phosphorylation of the RLC increases the mobility of the S1 head of myosin, bringing the S1 head in closer proximity to the myosin binding sites on actin. This increased mobility increases the rate of engagement of cross-bridges and increases the rate of force development and contraction magnitude on subsequent twitch or other submaximal contractions. We call this increased contractile response "activity dependent potentiation". With sequential twitches or incompletely fused tetanic contractions, the term staircase is used to describe the progressive increase in amplitude of contraction. If a twitch is elicited after a tetanic contraction, we call the enhanced response posttetanic potentiation. Stretching a muscle fiber to a longer length will also bring the actin filaments close to the myosin heads. This increases the Ca²(+) sensitivity, independent of RLC phosphorylation. At long sarcomere lengths, the impact of RLC phosphorylation is diminished, because Ca²(+) sensitivity is already increased. Similarly, lowering the temperature at which the muscle is tested increases Ca²(+) sensitivity. At low temperatures, staircase and posttetanic potentiation are diminished, but RLC phosphorylation still occurs. Activity dependent potentiation is an important functional modulator of contractile response.

Postactivation potentiation influences differently the nonlinear summation of contractions in young and elderly adults

The force enhancement of a twitch after a maximal conditioning muscle contraction [i.e., postactivation potentiation (PAP)] is reduced with aging, but its influence on the summation of force in response to repetitive stimulation at different frequencies is not known. The purpose of this work was to compare the electrically evoked mechanical responses of the tibialis anterior muscle between young and elderly adults after a 6-s maximal voluntary contraction (MVC). The results showed that, immediately after the conditioning MVC, twitch torque and its maximal rate of development and relaxation were significantly enhanced in both groups, but the magnitude of potentiation was greater in young (148.0 +/- 14.2, 123.7 +/- 16.5, and 185.4 +/- 36.5%, respectively) compared with elderly adults (87.4 +/- 15.2, 63.8 +/- 9.9, and 62.9 +/- 11.0%, respectively). This age-related difference in potentiation of the twitch disappeared completely 1 min after the conditioning MVC. The potentiation of torque and speed-related parameters in response to two- and three-pulse trains, delivered at a constant interval of 10 ms (100 Hz), was less than for a single pulse for both groups. In young adults, the magnitude of PAP on the successive individual mechanical contributions within a train of stimuli declined progressively such that the third contribution did not differ significantly from the same contribution before the conditioning MVC. In contrast, the second and third contributions did not potentiate (P > 0.05) in elderly adults. Although these contributions did potentiate significantly at a lower frequency of stimulation (20 Hz) in the two groups, the difference in PAP between young and elderly adults still persisted. This overall attenuation of potentiation with aging, however, appears to have a moderate influence on the decrement of the muscular performance.

Postactivation potentiation in human muscle is not related to the type of maximal conditioning contraction

The mechanical performance of a muscle can be enhanced by preceding contractile activity, such as occurs with postactivation potentiation. To investigate whether the type of contraction influences the extent of potentiation, the effects of 6-s maximal isometric (ISO), concentric (CON), and eccentric (ECC) maximal voluntary contractions (MVC) on the muscle twitch were compared in the tibialis anterior of nine subjects. The study also examined the effect of postactivation potentiation on the force evoked by the second (C2) and third (C3) responses of two-pulse (PT2) and three-pulse (PT3) trains that were delivered at a 10-ms interpulse interval. The results showed that immediately after the conditioning MVC, twitch torque (Pt) and its maximal rate of torque development (+dPt/dt) and relaxation (-dPt/dt) were significantly enhanced, without any change in contraction time (CT), half-relaxation time ((1/2)RT), and compound muscle action potential (M wave). The extent of Pt potentiation was similar for all MVC modalities, and the mean maximal values ranged from 150% to 180%. Furthermore, postactivation potentiation was greater for the single pulse compared with PT2 and PT3 responses. All parameters returned to initial values within 7-10 min. Although Pt (or C1) was potentiated more than was C2 and C3, its decline over time was proportionally more rapid than those for C2 and C3. We conclude that postactivation potentiation was not related to the type of conditioning MVC under these experimental conditions. The observation that postactivation potentiation increased C1 more than C2 and C3 indicates that a saturation process limits the extent of potentiation during the summation of successive responses to a train of stimuli. These results have practical application in the design of functional electrical stimulation protocols.

Nitric oxide mediates seasonal muscle potentiation in clam gills

The physiology and timing of gill muscle potentiation were explored in the clam Mercenaria mercenaria. When isolated demibranchs were exposed twice (with an intervening wash) to the same concentration of 5-hydroxytryptamine, the second contraction was larger than the first. This potentiation was seasonal: it was present from November through June, and absent from July through October. Potentiation was not affected by the geographic origin of the clams, nor by their acclimation temperature. Potentiation was inhibited by the nitric oxide synthase (NOS) inhibitor L-NAME and mimicked by the nitric oxide (NO) donor DEANO. During the season of potentiation, immunoreactive NOS appeared in the gill muscles and the gill filament epithelium, but during the off-season, the enzyme occurred at the base of the gill filaments. Potentiation was inhibited by ODQ, which inhibits soluble guanylate cyclase (sGC), and it was mimicked by dibutyryl-cGMP, an analog of cyclic GMP (cGMP). Moreover, potentiation was inhibited by the protein kinase G (PKG) inhibitor Rp-8-CPT-cGMPS. During the season of potentiation, immunoreactive sGC was concentrated in the gill muscles and the gill filament epithelium; but during the off-season, immunoreactive sGC was found in the gill filament epithelium. These data suggest that the potentiation of gill muscle is mediated by a NO/cGMP/PKG signaling pathway.

Increased phosphorylation of myosin light chain associated with slow-to-fast transition in rat soleus

In striated muscles myosin light chain (MLC)2 phosphorylation regulates calcium sensitivity and mediates sarcomere organization. Little is known about the changes in MLC2 phosphorylation in relation to skeletal muscle plasticity. We studied changes in MLC2 phosphorylation in rats receiving three treatment conditions causing slow-to-fast transitions: 1) atrophy induced by 14 days of hindlimb suspension (HS), 2) hypertrophy induced by 14 days of clenbuterol administration (CB), and 3) 14 days of combined treatment (CB-HS). Three variants of the slow (MLC2s) and two variants of the fast MLC2 (MLC2f) isoform were separated with two-dimensional electrophoresis and identified with monoclonal and polyclonal antibodies specific for MLC2; their relative proportions were densitometrically quantified. In control soleus muscle MLC2s predominated over MLC2f (91.4 +/- 3.9% vs. 8.5 +/- 3.9%) and was separated into two spots, the less acidic spot being 73.5 +/- 4.3% of the total. All treatments caused a decrease of the less acidic unphosphorylated spot of MLC2s (CB: 64.1 +/- 5.6%, HS: 62.4 +/- 6.8%, CB-HS: 56.4 +/- 4.4%), the appearance of a third more acidic variant of MLC2s (representing 3.9-5.9% of total MLC2s), an increase of MLC2f (CB: 30.9 +/- 3.1%, HS: 23.9 +/- 3.3%, CB-HS: 25.3 +/- 3.9%), and the phosphorylation of a large fraction of MLC2f (CB: 30.4 +/- 6.7%, HS: 28.7 +/- 6.5%, CB-HS: 21.8 +/- 2.1%). Treatment with alkaline phosphatase or with protein phosphatase 1 (PP1) removed the most acidic spots of both MLC2f and MLC2s. We conclude that in rat skeletal muscles an increase of MLC2 phosphorylation is associated with the slow-to-fast transition regardless of whether hypertrophy or atrophy develops.

Post-tetanic potentiation increases energy cost to a higher extent than work in rat fast skeletal muscle

We studied the effects of (post-tetanic) potentiation on myosin light chain (MLC-2) phosphorylation, work and energy cost in skeletal muscle. Experiments were performed using in situ medial gastrocnemius muscles of male Wistar rats, which were electrically stimulated through the severed sciatic nerve. One group of muscles was first potentiated with an isometric tetanus before a series of 10 concentric contractions (PRC). A second group performed the same series of contractions without previous potentiation (RC). Following the last contraction the muscles were rapidly frozen and excised after which the high-energy phosphate content, lactate concentration and the level of MLC-2 phosphorylation were measured. The results indicate that PRC muscles had a higher (P < 0.05) total work output 144.5 +/- 17.0 (SD) (n = 6) vs. 121.6 +/- 11.4 (SD) (n = 6) mJ and level of MLC-2 phosphorylation (49.2 +/- 7.3 vs. 40.8 +/- 3.6%) than RC muscles. The energy cost of the series of concentric contractions in the PRC muscles (9.8 +/- 1.9 micromol approximately P/muscle) was significantly higher (P < 0.05) than the energy cost in the RC muscles (6.2 +/- 0.97 micromol approximately P/muscle). It was shown that the relative increase in energy cost of PRC muscles was higher (P < 0.05) than in total work output. It is proposed that the relative high increase in energy cost is the direct result of the increase in muscle performance rather than a property of potentiation.